White Pigment Reflecting Ir Radiation, Production and Use Thereof

ABSTRACT

The invention relates to a pigment which reflects IR radiation, comprising an IR-reflecting core, the IR-reflecting core being provided with a substantially enveloping coating which is transparent to IR radiation, and in that the IR-reflecting pigment is substantially white. The invention further relates to a process for producing these pigments and also to their use.

The invention relates to largely white pigments which reflect IRradiation and also to their production and also to their use.

Wall paints are typically pigmented with white pigments such as titaniumdioxide or barium sulfate. On the basis of this white base paint,colored emulsion paints are obtained by tinting with corresponding colorpigments. The constituents of the emulsion paints, such as binders,pigments and/or fillers, for example, exhibit at least partialabsorption of IR radiation, however. Consequently the thermal radiationis not reflected and is ultimately emitted to the outside.

From K. Rose, U. Posset, K.-H. Haas and M. Köhl, Farbe & Lack 108 (2002)p. 29 it is known that aluminum pigments coated with SiO₂ allow improvedreflection of IR radiation in an emulsion paint.

In “Komfort und Energieeffizienz durch Wärmedämmung” [Comfort and energyefficiency through heat insulation], Prof. Dr. Beck, Otti-Profiforum“Wärmedämmung im Bauwesen” 09.+10, Mar. 2005, Regensburg, it isexpounded that interior room walls painted with wall paints comprisingIR-reflecting pigments generate a pleasant feel-good ambience. Theperson in a room acts as a blackbody radiator with a maximum of theirradiated IR radiation of about 10 μm. Although the IR radiationaccounts only for a fraction of a few percent of the total thermaleconomy of an interior room, subjectively the person reacts verysensitively to radiation sinks within a room. Consequently a uniformreflection of the IR radiation by the walls generates an extremelypleasant, “feel-good ambience”.

The use of, say, aluminum pigments as IR-reflecting pigments, however,has the disadvantage that the wall paint acquires a metallic appearance.In the majority of cases, however, this is unwanted. Metallic pigmentsin a wall paint, furthermore, significantly show up the unevennesses inthe substrate. In addition, depending on concentration and particlesize, the use of aluminum pigments has the disadvantageous effect of agraying of the emulsion paint. The majority of emulsion paints arewhite, and white emulsion paints are also used as a basis for tintingwith color pigments in order to produce colored emulsion paints.

DE 42 11 560 A1 discloses a coating of substrates, which among othersmay be metal flakes or mica pigments, with white pigments having aparticle size below 1 μm. The pigments are applied to the substratemerely by means of spray drying, without any further coating, andtherefore possess deficient adhesion to said substrate.

DE 100 10 538 A1 discloses a dirt-repelling coating material whichdiscloses a complex composition, which as well as a multiplicity ofcomponents and particles may also include platelet-shaped particles,such as aluminum pigments. One disadvantage of the coating materialknown from DE 100 10 538 A1 is that the metal pigments can corrode, andanother is that the coating material has a metallic appearance ormetallic effect.

DE 195 01 307 A1 discloses colored aluminum pigments wherein colorpigments are incorporated in a metal oxide matrix which is produced by asol-gel process. The resulting aluminum pigments are colored andmetallically lustrous.

U.S. Pat. No. 5,037,475 likewise discloses colored aluminum pigmentscoated with color pigments. In this case the attachment of the colorpigments is on the one hand via a thermally polymerized, unsaturated,polyfunctional carboxylic acid and on the other hand via a plasticcoating. A disadvantage, again, is that the colored aluminum pigmentsthus produced have a distinctly metallic appearance.

In addition, WO 91/04293 discloses colored and metallically lustrousmetal pigments.

In the case of the prior art identified above, the descriptions arealways of effect pigments for the decorative sector. These pigments havenot only a metallic effect but also a masstone, since the color pigmentsare fixed directly on the metallic surface.

WO 96/23337 discloses a coating material, featuring two particles—in onecase platelet-shaped metal pigments and in the other case whitepigments—which have a very high absorption in the near infrared range.The white pigments may also have been applied to the metal pigments. Notdisclosed, however, is how the white pigments are fixed on the metalpigments.

WO 2005/007754 discloses colored pigments which have aninfrared-reflecting core with a thickness below 0.2 μm. In this case nowhite pigments are disclosed, and nor is it disclosed how they are fixedon the metal pigments.

DE 40 35 062 A1 discloses an IR-reflecting substrate coated with avarnish layer which may comprise white, gray, black or chromaticpigments. Not disclosed herein are emulsion paints which can be appliedto walls in the habitual way.

In accordance with the teaching of DE 197 18 459 A1 the intention is touse metal pigments having a whitish to grayish surface. This whitish tograyish surface is to be producible by means of various chemicalreactions, although DE 197 18 459 A1 does not reveal which reactionsmust be carried out under which conditions in order to obtain the statedmetal pigments.

There is therefore a need for IR-reflecting pigments, preferably metalpigments, which have a white appearance and in which any metallic effectis suppressed. A further object is to provide IR-reflecting pigments,preferably metal pigments, which, when used in an application medium,such as a paint or a varnish, for example, are not markedly visible tothe human eye and do not lead to any substantial graying of theapplication medium.

It is a further object of the invention to provide an IR-reflectingpigment which is stable to corrosion in respect of the influence ofwater and alkalis. The pigment ought to be amenable to use in emulsionpaints, not only in an interior wall paint but also in a masonry paint.

It is an object of the present invention, furthermore, to provide acost-effective process for the production of such pigments.

The object on which the invention is based is achieved by means of apigment which reflects IR radiation, comprising an IR-reflecting core,the IR-reflecting core being provided with a substantially envelopingcoating which is substantially transparent to IR radiation, and theIR-reflecting pigment being substantially white.

Preferred developments of the pigment of the invention are specified independent claims 2 to 23.

The object is further achieved by means of a process for producing anIR-reflecting pigment of any one of claims 1 to 23, where a coatingwhich is substantially transparent to IR radiation, together with whitepigments and/or with particlelike coating outgrowths that scattervisible light, is applied to an IR-reflecting core.

Preferred developments of the process of the invention are specified independent claims 25 to 27.

The object on which the invention is based is further achieved throughthe use of the pigment of the invention in inks, paints, varnishes,printing inks, security-printing inks, and cosmetics.

A preferred development is specified in dependent claim 29.

The object is also achieved by means of a coating composition whichcomprises a pigment of the invention according to any one of claims 1 to23.

One preferred development is specified in dependent claim 31.

The object is further achieved by an article which is coated with apigment of the invention according to any one of claims 1 to 23 or witha coating composition of the invention according to either of claims 30and 31.

The article may be, for example, a coated wall material or ceilingmaterial, a coated building material, such as façade material, forexample, etc.

The inventors have surprisingly found that it is possible to provide apigment which has an IR-reflecting core and at the same time appearssubstantially white to the human eye.

A particular surprise in this context was that an IR-reflecting core,such as a substrate having a metallic surface which reflects IRradiation extremely effectively, for example, can be coated in such away that, on the one hand, the IR reflection capacity is notsubstantially impaired and, on the other hand, the pigment appearslargely white and nonmetallic to the human eye.

The pigment of the invention can therefore be used in white applicationmedia, such as inks, paints, varnishes or cosmetics, for example,without there being any distinctly marked graying or any metallic lusteror any strong sparkle effect on the part of the application medium.

Consequently the pigment of the invention is suitable more particularlyfor use in white emulsion paints which are typically used for paintinginterior room walls. It will be appreciated that emulsion paints of thiskind comprising the pigments of the invention can also be tinted in atypical way through addition of further colorants.

For the production of the pigment of the invention, an IR-reflectingcore is given a substantially uniform coating of pigmentlike particleswhich are opaque in the optical wavelength range and at the same timeare largely IR-transparent. A core for the purposes of the invention isa particulate, preferably spherical or platelet-shaped, substrate. Withvery particular preference the substrate possesses a platelet-shapedform, since this geometric morphological form combines the greatest IRreflection with the least amount of material, i.e., a relatively lowlevel of pigmentation.

The coating is preferably composed of the substantially IR-transparentpigment particles (white pigments) on the one hand and of a matrixmaterial on the other hand. The substantially IR-transparent pigmentparticles may be fixed on the surface of the IR-reflecting core, bybeing incorporated in and/or on an optically transparent matrix, forexample. This matrix provides preferably uniform envelopment of thecore. This preferably enveloping matrix also protects the core againstthe corrosive effect of water or atmospheric gases.

A substantially enveloping coating for the purposes of the inventionmeans that the IR-reflecting core is enveloped by the coating in such away that, to a viewer, the core does not produce any perceptiblelustrous impression. Furthermore, the degree of envelopment is so largethat, in the case of a corrosion-susceptible metallic IR-reflectingcore, the incidence of corrosion is suppressed or prevented.

As a result of the uniform coating of the IR-reflecting core withpigmentlike particles which are opaque in the optical wavelength rangeand at the same time are largely IR-transparent, the pigment of theinvention overall acquires a largely white appearance. The opticaleffect originating from the IR-reflecting core is largely suppressed.Owing to the large IR transparency of the pigmentlike particles,surprisingly, the IR reflection capacity of the core is not—or notsubstantially—adversely affected.

In the context of this invention, “optical properties” or “opticaleffect” are always those properties of the IR-reflecting pigments thatare visible to the human eye. Physically, these properties aredetermined substantially by the optical properties in the wavelengthrange from approximately 400 to approximately 800 nm.

In one preferred variant the white, optically opaque and IR-transparentpigmentlike particles are white pigments having an average primaryparticle size of preferably 180 to 400 nm, more preferably of 250 to 350nm, and more preferably still of 270 to 330 nm. According to the Mietheory, pigments of this kind possess the greatest scattering crosssection for electromagnetic wavelengths in the optical range from 400 to800 nm. Both smaller and larger white pigments have far lower scatteringproperties. The lowest scattering is produced, for example, bynanoparticles having a primary particle size below from 30 to 40 nm,which are almost completely transparent. The fact that the particlespreferably have a particle size with the greatest scattering crosssection has the effect that they appear substantially white and theoptical light almost do not reach the surface of the IR-reflecting core.Consequently the optical effect of the core, the metallic effect forexample, is substantially suppressed and the overall pigment appearssubstantially white.

The white pigments may be selected, for example, from the groupconsisting of titanium dioxide, zinc oxide, magnesium oxide, zincsulfide, calcium fluoride, lithium fluoride, sodium fluoride, potassiumfluoride, calcium carbonate, lithopones, magnesium carbonate, bariumsulfate, barium titanate, barium ferrite, and mixtures thereof. Thewhite pigment is substantially transparent, preferably transparent, toIR radiation. The particle size, and also the amount of white pigmentapplied to the IR-reflecting core, are adjusted as a function of thewhite pigment used.

Preference is given to using TiO₂ in the rutile or anatase modification,barium sulfate, zinc oxide and/or zinc sulfide, with particularpreference being given to TiO₂ and ZnO on account of their universalavailability in all sizes. TiO₂ in the rutile form has proven verysuitable.

By a coating which is substantially transparent to IR radiation ismeant, for the purposes of the invention, that only a small fraction ofIR radiation is absorbed by the coating and/or the white pigments.Together with the IR-reflecting properties of the core, this leads to ahigh IR reflectance. The IR reflectance ρ_(IR) as a function of thetemperature T can be calculated from the spectral reflectance R(λ) byintegration over all wavelengths, with the Planck function i(T) as aweighting function:

$\begin{matrix}{{\rho_{IR}(T)} = \frac{\int_{1.4}^{35}{{R(\lambda)}*{i(T)}*\ {\lambda}}}{\int_{1.4}^{35}{{i(T)}*\ {\lambda}}}} & (1)\end{matrix}$

The Planck function i(T) indicates how much a blackbody would emit at agiven temperature T. The relevant spectral range for room temperaturecorresponds in good approximation to the wavelength range from 1.4 to 35μm.

IR-reflecting pigments of the invention, in the wavelength range from2.5 to 25 μm and at a calculated temperature of 300 K, have an IRreflectance of preferably more than 50%. With further preference the IRreflectance is at least 60% and with even further preference at least70%. Very preferably the IR reflectance is at least 80%, and mostpreferably it is at least 85%.

The spectral IR reflectivity of the pigments of the invention can bedetermined by means of a diffuse reflection measurement in a KBr powderbed, as follows. First of all KBr powder is comminuted in a mortar. Thenpigment is added to the KBr powder to a concentration of 1.5% by weight,and the constituents are combined homogeneously with one another. Atablet-shaped sample chamber (diameter: about 0.8 cm, depth: about 2.2mm) is filled with the pigment/KBr mixture, which is tamped down.Subsequently the diffuse reflection is measured in a wavelength rangefrom 2.5 to 25 μm. This is done using, as a measuring unit, the Selector(from Specac). This instrument measures the diffuse IR reflection in aquarter-sphere geometry. The IR instrument used is an Avatac 360spectrometer from Thermo; the detector is a DTGS detector. In every casea pure KBr powder is measured as the background spectrum, and thespectrum of the pigmented KBr is compared against it. The procedure wasrepeated three times and the average value of the measurements wastaken.

From the IR reflection spectrum that is obtained it is possible tocalculate the IR reflectance ρ_(IR) by formula (1), using a temperatureof 300 K with the Planck function.

In order to provide further characterization of the absorption of thesubstantially transparent and substantially enveloping coating, theabove-described IR reflectance of a coated pigment of the invention canbe related in percent terms to the IR reflectance of the uncoatedIR-reflecting core (1.5% by weight concentration). This ratio isreferred to in the context of this invention as “IR reflectance,coating”. The ratio is preferably above 65%, more preferably above 70%,and with particular preference above 80%. With further preference thisratio is above 85% and even more preferably above 90%. As an upper limitthe ratio is 99%.

The term “a substantially or largely transparent enveloping coating”refers to those coatings for which the IR-reflecting pigment of theinvention has the above-specified properties in terms of its IRreflectance. The substantially or largely transparent enveloping coatingpreferably features the white pigments which give rise to or improve thewhite appearance.

The white pigments used may also have been surface-treated and may havebeen coated, for example, with metal oxides. In particular it ispossible for TiO₂ pigments to have coatings of, for example, SiO₂, Al₂O₃and/or manganese oxides and/or cerium oxides, in order to suppress thephotoactivity of the TiO₂ pigments. Advantageously, however, thephotoactivity of the TiO₂ pigments is suppressed by the envelopingmatrix itself by which the TiO₂ pigments are fixed to the surface of theIR-reflecting core.

The amount of white pigment used is dependent on the type and size ofthe pigment and in particular on the specific surface area of theIR-reflecting core. The specific surface area of the IR-reflecting coreis the surface area of the IR-reflecting core per unit weight. Thespecific surface area of the IR-reflecting core is determined by theknown BET method.

In order to ensure sufficiently high whiteness of the IR-reflectingpigments of the invention they preferably have white pigments in anamount from 20% to 80% by weight, more preferably from 35% to 70% byweight, and with particular preference from 40% to 60% by weight, basedin each case on the weight of the total IR-reflecting pigment. In thiscontext it is preferred to use an amount of approximately 20% by weightin the case of IR-reflecting cores having low specific surface areas,and an amount of about 80% by weight in the case of IR-reflecting coreshaving high specific surface areas.

With amounts of below 20% by weight of white pigments, the whiteness ofthe IR-reflecting pigments may be too low. At amounts of more than 80%by weight there may be inadequate IR reflection, since the fraction ofthe IR-reflecting core, based on the total pigment, may be too low. Inorder to obtain effective IR reflection with the latter pigments in,say, an emulsion paint, that emulsion paint must have a correspondinglyhigh level of pigmentation. High pigmentation, i.e., a high level ofpigment of the invention in the application medium, leads on the onehand to high production costs. On the other hand it may also result inoverpigmentation and hence in poor performance properties on the part ofthese emulsion paints.

IR-reflecting cores used are preferably metal powders and/orplatelet-shaped metal pigments and/or suitable pearlescent pigments.Particular preference is given in this context to platelet-shaped metalpigments, since on account of their shaping and their opticalproperties, in the case of preferably plane-parallel orientation in theapplication medium, they exhibit the highest IR reflection. The metalpigments are opaque both to optical light and to IR radiation. Even onnonplanar substrates, such as woodchip wallpapers, for example,platelet-shaped metal pigments bring about the most effective directedand/or diffuse reflection of incident IR radiation.

Platelet-shaped metal pigments or metal powders employed are preferablyaluminum, copper, zinc, titanium, iron, silver and/or alloys of thesemetals. Particular preference is given to aluminum and to alloys ofaluminum, on account of their extremely high IR reflection and the readyavailability of these metal pigments. The platelet-shaped metal pigmentsare also referred to in accordance with the invention as metallic effectpigments.

The dimensions of the length and width of the platelet-shaped pigments,preferably metal pigments or metallic effect pigments, are preferablybetween 3 and 200 μm, more preferably between 12 and 90 μm, morepreferably still between 20 and 75 μm, and with particular preferencebetween 40 and 70 μm.

The average thickness of the platelet-shaped pigments, preferably metalpigments or metallic effect pigments, is preferably between 0.04 and 4μm, more preferably between 0.1 and 3 μm, and with particular preferencebetween 0.3 and 2 μm.

The platelet-shaped pigments, preferably metallic effect pigments,preferably have specific surface areas of about 0.2 to about 15 m²/g.Metal pigments or metallic effect pigments with a length or width below3 μm exhibit excessive scattering in the optical range and thereforeappear too gray even after coloring with a white pigment. Moreover,pigments of this size no longer provide optimum reflection of the IRradiation, since in this case the pigments are already smaller than thewavelength of the IR light to be reflected. Furthermore, on account oftheir high specific surface area, these metal pigments or metalliceffect pigments can no longer be fully coated with white pigments or canno longer tie the white pigments correspondingly into a coating. Above alength or a width of 200 μm, the opacity achieved by the pigments inrespect of the IR-reflecting metal fraction, and hence the IRreflection, in an applied wall paint or a varnish, for example, isinadequate. Moreover, in spite of their white appearance, pigments withsizes of more than 200 μm can be perceived even to the eye as particles,which is undesirable. Furthermore, in the case of a pigment size of morethan 200 μm, agglomerates, and hence the formation of bits, are likely.

The platelet-shaped metal pigments may be present in a prepassivatedform. Examples of such are SiO₂-coated aluminum pigments (Hydrolan®, PCXor PCS®, Eckart) or chromated aluminum pigments (Hydrolux®, Eckart).Using substrates prestabilized in this way maximizes the stabilities interms of the gassing stability in an aqueous paint, more particularly anemulsion paint, and also, possibly, the corrosion stabilities in theexterior sector.

In the case of another preferred embodiment, the platelet-shapedpigments, preferably metal pigments, possess lengthwise dimensions of 5to 12 μm. Pigments of this kind are used predominantly as white, opaquepigments. In this case relatively small metal pigments are used as thecore in order to obtain high opacity.

In another preferred case of platelet-shaped metal pigments, and moreparticularly of aluminum pigments, as the IR-reflecting core, the amountof white pigment applied to the preferably platelet-shaped metalpigment, per 1 m² surface area of the IR-reflecting metal core, ispreferably 0.3 to 10 g, more preferably 0.5 to 7 g, with further,particular preference 1 to 3 g, and with particular preference 1.5 to2.5 g.

Below 0.3 g/m² substrate surface area, the coating of the preferablyplatelet-shaped metal pigment with the white pigment may be too low toimpart a satisfactory white effect. Above 10 g/m² the white effect ispractically saturated and the fraction of the IR-reflecting core as aproportion of the total pigment may be too low, with the consequencethat a pigment of the invention of this kind may no longer exhibitsufficient IR reflectivity.

As IR-reflecting cores it is additionally possible to use metal powders.Suitable powders preferably have an approximately spherical morphologywith an average diameter of preferably 8 to 1000 μm, more preferably 10to 500 μm, and with particular preference 20 to 300 μm. Irregularlyshaped metal particles, however, can also be used as IR-reflectingcores.

As IR-reflecting cores it is also possible to use pearlescent pigments.These pearlescent pigments preferably have a low-refractive-index core,such as mica, glass, SiO₂ or Al₂O₃ flakes, coated with high-index oxidessuch as TiO₂ and/or Fe₂O₃. Examples of SiO₂ flakes coated with TiO₂and/or Fe₂O₃ are known under the Colorstream® name, and examples ofcorresponding Al₂O₃ flakes under the Xirallic® name, and are bothproduced by Merck, Darmstadt, Germany. Given appropriate opticalthicknesses of the high-index coat, the interference conditions resultin large reflection of the IR radiation. The suitable opticalthicknesses are set as a function of the refractive indices of thehigh-index oxide.

The pigments of the invention preferably possess high reflection in theIR range from 4 to 25 μm, more preferably from 5 to 15 μm, and withfurther preference from 8 to 12 μm. It has emerged that an optimumfeel-good ambience in the interior of a room is generated if there arehigh reflections, preferably even reflection maxima, within theaforementioned ranges, since in that case the pigments of the inventioneffect optimum reflection of a person's IR radiation. Accordingly thepigments of the invention are suitable more particularly for use forwall paints which are applied in interior rooms.

Furthermore, extremely advantageously, rooms whose interior walls havebeen provided with the pigments of the invention do not have to beheated so greatly in winter, owing to the IR reflectivity of thepigments of the invention. Accordingly the pigments of the inventionmake it possible to save energy, which signifies a great advance fromboth an environmental and an economic standpoint, in view ofincreasingly scarce energy resources and continually increasing energycosts.

Examples of pearlescent pigments which can be used as IR-reflectingcores are the pearlescent pigments produced by Merck, Darmstadt, Germanyand sold under the brand names Solarflair® or Minatec®.

Using platelet-shaped cores as an IR-reflecting component in thepigments of the invention optimizes IR reflection in relation to theamount of IR-reflecting material used. In the case of platelet-shapedcores, a very great improvement is obtained, on the one hand, in theopacity properties of pigments, preferably of metal pigments, ascompared, for example, with spherical pigments such as metal beads, forexample. On the other hand, in comparison to spherical pigments, thereflectivity of platelet-shaped pigments, preferably metal pigments, isgreater by virtue of the greater reflection area.

As IR-reflecting cores it is preferred to use effect pigments, examplesbeing metallic effect pigments or pearlescent pigments, since thesepigments, on account of their platelet form, are particularly suitablein respect of opacity and reflectivity in the context of the presentinvention.

Effect pigments typically have an optical appearance which is dependenton the angle of incidence and/or viewing. The optical effects mayencompass changes in lightness in the case of metal pigments, which arealso referred to as “flop”, and color changes in the case of pearlescenteffect pigments, which are also referred to as “color flops”.

A typical feature of application media, such as inks, paints orvarnishes, which comprise effect pigments are their high gloss values.Metal powders, in contrast, in application media such as inks, paints orvarnishes, always give rise in optical terms to severe graying, whichgoes hand in hand with low lightness.

These optical effects which typically occur when effect pigments areused, pearlescent pigments or metallic effect pigments for example, orwhen metal powders are used are largely suppressed in the case of thepigments of the invention. To the human eye the pigments of theinvention have a largely white appearance.

It has emerged that, when pearlescent pigments are used as anIR-reflecting core, the desired whiteness is relatively easy to achievein comparison to metallic effect pigments. With metallic effectpigments, however, the IR reflection is far higher. Consequently,metallic effect pigments are preferred as an IR-reflecting core in thepigments of the invention, despite the fact that the amount ofIR-transparent white pigments for application to the metal pigments isgreater. Platelet-shaped aluminum pigments are particularly preferred inthis context.

A largely white IR-reflecting pigment of the invention that has nomarked metallic effect to the human eye or has a color effect which isdifferent from the color white, preferably meets the following criteria:

With the pigment of the invention, preferably a metallic effect pigment,the parameters of gloss, chroma C*, flopindex, and the lightness L*,measured in each case at a constant incident angle of 45°, arepreferably situated within defined ranges of values. Even the sparkleeffect that frequently occurs in the case of effect pigments is largelysuppressed. This sparkle effect, however, cannot be measured bycolorimetry and can therefore only be assessed visually.

In order to be able to determine these parameters comparatively, theapproach taken is as follows: on the one hand, the pigments of theinvention are incorporated into an otherwise unpigmented conventionalvarnish based on a polyester/CAB system (binders: 22% by weight CAB381-2 and 9% by weight CAB 551-0.2, both from Eastman, and 13% by weightViacryl SC 303, from Surface Specialties. No other pigments or mattingagents are added to this varnish, referred to below as the “testvarnish”, since they would influence the parameters it is intended todetermine, more particularly gloss and chroma. The level of pigmentationchosen is 10% by weight, and the pigment-containing varnish isknife-coated on a black substrate. The coating knife depth is 50 μm, andin the case of very coarse pigments is 100 μm.

These knife drawdowns are used to determine the chroma, lightnessvalues, and flop value in the context of the CieLab color system.Measurement is carried out using a multiangle colorimeter, an examplebeing the M 682 from X-Rite, in accordance with manufacturer'sindications, with a constant incident angle of 45° and with differentviewing angles relative to the specular angle, and the L* and C* valuesare ascertained. Relevant more particularly are the viewing angles at15°, 25°, 45°, and 110°.

For the evaluation of the color saturation, referred to as the chroma,the value to be employed is C*₂₅°.

The C*₂₅° value of the knife drawdowns of the pigments of the inventionis preferably within a range from 0.0 to 2.5, more preferably from 0.1to 1.0. Values of this kind are achieved only by virtually colorlesspigments.

Since in principle even certain achromatic metal pigments and alsosilver pearlescent pigments can achieve C*₂₅° values of this kind, it ispreferred to employ further parameters for characterizing the pigmentsof the invention.

For the assessment of the lightness L*, in this case the value at 45° isemployed. In terms of their lightness, effect pigments are frequentlycharacterized by values close to the specular angle, i.e., at 15° or20°. The pigments of the invention exhibit a largely angle-independentlightness, i.e., they have no significant lightness flop. A moreeffective differentiation from pure metal pigments or metal powders istherefore achieved in the case of median values.

The L*₄₅° values of the pigments of the invention are preferably 50 to90 units, more preferably 55 to 80 units, and more preferably still 60to 75 units.

The lightness flop is specified by DuPont in accordance with thefollowing formula (A. B. J. Rodriguez, JOCCA, (1992(4)) pp. 150-153):

$\begin{matrix}{{{Flop}\mspace{14mu} {index}} = {2.69 \times \frac{\left( {L_{15{^\circ}}^{*} - L_{110{^\circ}}^{*}} \right)^{1,11}}{\left( L_{45{^\circ}}^{*} \right)^{0.86}}}} & (2)\end{matrix}$

The flop index shows the characteristic lightness flop more particularlyof metallic effect pigments, and is less applicable to pearlescentpigments or metal powders.

The pigments of the invention possess a lightness flop of 0 to 3,preferably of 0.1 to 2, and more preferably of 0.15 to 1.0.

These extremely low values show that, for example, in the case ofmetallic effect pigments, the otherwise so typical lightness flop, witha flop index in a range from approximately 4 to approximately 25, islargely or completely suppressed in the case of the pigments of theinvention.

One characteristic more particularly of effect pigments is the highgloss of the ink, paint or varnish coating comprising the effectpigments. Since the pigments of the invention no longer exhibit thesecharacteristic optical gloss properties of effect pigments, thedrawdowns possess very low gloss values.

The criterion employed here is the gloss at 60°, which was measuredusing a Trigloss instrument from Byk-Gardner, Germany, in accordancewith the manufacturer's instructions. The pigments of the inventionpossess a gloss of 1 to 12, preferably of 1.5 to 10, units. With effectpigments the gloss is typically situated within a range fromapproximately 30 to 160.

A further criterion for the largely white appearance of the pigment ofthe invention can be determined on the basis of its appearance in acommercially available white emulsion paint. In this case a comparisonis made between the pigments of the invention and the IR-reflectingpigments with no coating and/or with no white pigment coating. Thelightness of correspondingly pigmented emulsion paints is measured indiffuse reflection. The level of pigmentation of the IR-reflected corein this case is 10% by weight, based on the total emulsion paint.Subsequently the difference of the corresponding lightnesses is formed:

ΔL*diffuse=L* _(diffuse, white pigment) −L*_(diffuse, no white pigment).

This difference ought preferably to be greater than 1.5 units,preferably greater than 3 units, and more preferably greater than 7units.

This difference in lightness, however, cannot be used alone to assessthe whiteness. For instance, metal pigments always produce a sharpincrease in lightness, without being therefore white. Ultimately here,therefore, it is still always the visual impression which is decisive.

The coating substantially transparent to IR radiation that largelyenvelopes not only the core but also the white pigments constitutes amatrix which is largely colorless from an optical standpoint. Itcomprises or consists preferably of metal oxides and/or organicpolymers. The white pigments may also be applied on the envelopingcoating or matrix. The matrix is preferably largely colorless, in ordernot to adversely affect the white effect produced by the applied orincorporated white pigments.

By largely colorless is meant, in accordance with the invention, thatthe metal oxides and/or organic polymers have no substantial inherentcoloration that cannot be masked by the white effect generated by thewhite pigments.

If the core is composed of a metal pigment, then the largely colorlessmatrix material is preferably a metal oxide, since in this way the corecan be protected very well from corrosion. The metal oxide to be usedfor the matrix material, and the amount of that oxide, are selected moreparticularly under the consideration that the pigment of the inventionshould absorb IR radiation to as small an extent as possible. Any IRabsorption on the part of the pigments of the invention results inreduced IR reflection and hence weakens the desired effect of thepigments of the invention. The matrix material brings about adhesion ofthe white pigments on the IR-reflecting core, and so, even afterdispersion into the emulsion paint, the white pigments remain largelyadhering to the IR-reflecting core. It is only this reliable attachmentthat allows the optical phenomena typical of effect pigments to besuppressed, and permits the largely white appearance.

Examples of very suitable metal oxides are titanium dioxide, silicondioxide, aluminum oxide/hydroxide, boron oxide/hydroxide, zirconiumoxide or mixtures thereof. Silicon dioxide is particularly preferred.

As organic polymers it is preferred to use those which are also employedas binders in varnishes, emulsion paints or printing inks. Examplesthereof are polyurethanes, polyesters, polyacrylates and/orpolymethacrylates. It has emerged that the effect pigments of theinvention can be incorporated very effectively into binders if theorganic coating and the binder are very similar to one another oridentical.

The optically largely colorless matrix is present preferably in afraction of 4% to 40% by weight, based on the weight of the totalpigment. The fraction is preferably 5% to 20% and more preferably 6% to15% by weight. Surprisingly it is possible, with such small amounts ofmatrix material, not only to anchor the white pigments firmly anduniformly on the surface of the cores but also, in the case of metalliccores, to achieve corrosion stability on the part of these cores. At afraction below 4% by weight it may be the case that the pigments are notanchored with sufficient firmness on the surface of the IR-reflectingcore. It may be the case, moreover, that, with metallic cores, therequisite corrosion stability, which requires very substantiallycomplete envelopment of the cores with the matrix, is not sufficientlyprovided at these low quantities. In the case of amounts above 40% byweight it may be the case that not only the IR reflection but also thewhiteness of the pigments are too low. It may be the case, furthermore,that the IR absorption undergoes an unfavorable increase as a result ofthe matrix material.

In one particularly preferred embodiment the IR-reflecting core iscomposed of aluminum and the optically largely colorless matrix iscomposed of SiO₂. In addition it is preferred for the white pigment tobe TiO₂ ZnS and/or ZnO, which preferably have an average primaryparticle size of 250 to 370 nm and with particular preference of 250 to320 nm.

Aluminum possesses the highest IR reflection and is very readilyavailable commercially. SiO₂ is outstandingly suitable for providing thealuminum with corrosion stabilization, and TiO₂, on account of its highrefractive index, is a very good white pigment, and is likewise veryreadily available commercially. Furthermore it has been found,surprisingly, that ZnS particles or ZnO particles, more particularlyhaving a primary particle size in the range from 250 to 370 nm and withparticular preference from 250 to 320 nm, absorb IR radiation hardly atall and are therefore especially suitable in the context of the presentinvention.

In accordance with a further preferred embodiment the pigments of theinvention have an organic surface modification. The pigments of theinvention are preferably modified with leafing promoter agents. Theleafing promoter agents produce floating of the pigments of theinvention on the surface of the application medium, an ink or paint forexample, preferably an emulsion paint, a varnish or a cosmetic. Byvirtue of the fact that the pigments of the invention are ordered at thesurface of the application medium, the IR reflectivity in the appliedstate is improved, since the IR radiation is reflected right at thesurface of the application medium and does not first have to penetratethe application medium, in which case there may be absorption losses.

The pigments of the invention are preferably surface-modified withlong-chain saturated fatty acids such as stearic acid, for example, orpalmitic acid or with long-chain alkylsilanes having 8 to 30 C atoms,preferably 12 to 24 C atoms, or with long-chain phosphoric acids orphosphonic acids or their esters and/or with long-chain amines.

In the case of a further embodiment according to the invention thepigmentlike particles are composed not of individual, commerciallyavailable white pigments but instead of particlelike outgrowths of acoating material having a refractive index >2.0. In this case thecoating may be composed first of a smooth layer of this high-indexmaterial, but which then, in terms of its morphology, increasinglyadopts a particulate form on the side of the coating facing away fromthe IR-reflecting core. Forms of this kind may be represented, forexample, in a kind of “cauliflower structure” if the pigments areinvestigated by scanning-electron methods. Preference is given here tolayers and particlelike coating outgrowths of TiO₂.

The pigments of the invention can be produced by applying a coatingwhich is substantially transparent to IR radiation, together with whitepigments and/or particlelike coating outgrowths that scatter visiblelight, to an IR-reflecting core.

The coating preferably envelopes the IR-reflecting core substantiallycompletely, with further preference completely. The white pigments whichare substantially transparent to IR radiation and/or particlelikecoating outgrowths that scatter visible light are applied in and/or onthe coating.

In order to avoid repetition in respect of the pigment of the inventionproduced in accordance with the processes of the invention, reference ismade to the above elucidations, which apply correspondingly to theprocess of the invention.

In one preferred variant of the process the white pigments that aresubstantially transparent to IR radiation are precipitated envelopinglyaround the core together with metal oxide, using wet-chemical sol-gelprocesses, with the consequence that the white pigments aresubstantially imbedded in the metal oxide layer.

One preferred variant of the process encompasses the following steps:

-   -   a) dispersing the platelet-shaped IR-reflecting pigment core in        a solvent, preferably in an organic solvent;    -   b) adding water, a metal alkoxy compound and, if desired,        catalyst, with optional heating in order to accelerate the        reaction;    -   c) adding the IR-transparent white pigment, preferably in the        form of a dispersion in a solvent, more preferably in organic        solvent.

After the end of the reaction the pigment of the invention, i.e., theplatelet-shaped IR-reflecting core coated with white pigments and metaloxide, can be separated from unreacted starting materials and from thesolvent. After that it is possible for drying and, optionally, sizeclassification to take place.

As a metal alkoxy compound it is preferred to use tetraalkoxysilanessuch as tetramethoxysilane or tetraethoxysilane, in order to effectprecipitation of an SiO₂ layer, with white pigments preferably imbeddedin it, onto and preferably enveloping the core.

As organic solvents it is preferred to use water-miscible solvents.Particular preference is given to using alcohols such as, for example,methanol, ethanol, n-propanol, isopropanol, n-butanol, isobutanol ortert-butanol.

The amount of water ought preferably to be between 1.5 times and 30times the amount required by stoichiometry for the sol-gel reaction. Theamount of water is preferably between 2 times and 10 times thestoichiometrically required amount.

Below 1.5 times the stoichiometrically required amount, the reactionrate of the sol-gel process is too slow, and above 30 times thestoichiometrically required amount the formation of a coat may not besufficiently uniform.

The reaction temperature during the sol-gel reaction is preferablybetween 40° C. and the boiling temperature of the solvent used.

In the context of the sol-gel reaction it is possible as catalysts touse weak acids or bases.

Acids used are preferably organic acids such as acetic acid, oxalicacid, formic acid, etc., for example.

Bases used are preferably amines. Examples thereof are as follows:ammonia, hydrazine, methylamine, ethylamine, triethanolamine,dimethylamine, diethylamine, methylethylamine, trimethylamine,triethylamine, ethylenediamine, trimethylenediamine,tetramethylenediamine, 1-propylamine, 2-propylamine, 1-butylamine,2-butylamine, 1-propylmethylamine, 2-propylmethylamine,1-butylmethylamine, 2-butyl-methylamine, 1-propylethylamine,2-propylethylamine, 1-butylethylamine, 2-butylethylamine, piperazineand/or pyridine.

The white pigments can be comminuted mechanically, preferably prior tothe addition to the coating suspension, in order to have as many primaryparticles present as possible. This may take place, as is typical, in anorganic solvent, where appropriate with the addition of suitabledispersing additives and/or binders. The comminution may take place inthe typical assemblies, such as a triple-roll mill, co-ball mill,toothed-wheel dispersing mill, etc, for example.

In the case of another embodiment of the process of the invention thepigments of the invention are produced by a spray-drying process.

With this variant of the process a dispersion comprising a highlyvolatile, preferably organic, solvent, IR-reflecting cores,IR-transparent white pigments with an average size of preferably 180 to400 nm, and an organic, preferably film-forming, binder together issprayed and is dried in the course of the spraying. The spray drying iscarried out preferably in an agitated atmosphere, such as in a fluidizedbed, for example, in order to prevent agglomeration. In the course ofspray-drying, the cores are coated uniformly with the organic,preferably film-forming, binder and with the white pigments. Afterdrying, the organic, preferably film-forming, binder can be cured. Thiscan be done preferably likewise in the spray-drying apparatus, by means,for example, of the temperature of the feed gas being above the curingtemperature of the binder.

In another embodiment of the process of the invention the IR-reflecting,preferably platelet-shaped, pigment of the invention can be obtained bycoating the IR-reflecting cores with a matrix comprising suitablestarting compounds and white pigments in a fluidized-bed process.

The IR-reflecting, preferably platelet-shaped, pigments of the inventionare used preferably in inks, paints, varnishes, printing inks,security-printing inks, and cosmetics.

The IR-reflecting, preferably platelet-shaped, pigments of the inventionare used preferably in emulsion paints for the interior or exteriorsector.

Application media pigmented with the pigments of the invention, emulsionpaints for example, possess a largely white appearance. The whiteness ofthese application media may where appropriate be increased further bymeans of further addition of white pigments such as TiO₂ or else offillers. Furthermore, by tinting with colorants such as organic orinorganic color pigments, it is possible to produce colored emulsionpaints.

In order to maximize the IR emittance of a wall painted, for example,with an emulsion paint, it is preferred for the emulsion paint tocontain the pigments of the invention in an amount such that thefraction of the IR-reflecting cores, based on the weight of all thenonvolatile components of the emulsion paint, is 2% to 30%, preferably4% to 20%, and more preferably 7% to 15% by weight.

With emulsion paints pigmented with the pigments of the invention,coatings are possible which possess IR emittances of below 0.5,preferably below 0.4, and more preferably below 0.3. The lower limit ofthe emission in this case is approximately 0.2.

The degree of IR emittance is defined as follows:

emittance=1−reflectivity  (3)

For comparison with this, a conventionally applied wall paint possessesan emittance of approximately 0.9; in other words, only about 10% of theIR radiation is reflected, and 90% of the IR radiation is absorbed ortransmitted by the wall paint and, ultimately, is lost in the form ofheat.

In order to be able to minimize emittances or maximize reflectances itis preferred for the further components of the emulsion paint, such asbinders or fillers, for example, likewise to have a very low IRabsorption. In addition, owing to the additional pigmentation by thepigments of the invention, the levels of pigmentation of the binders,fillers and/or white pigments may be significantly lower than is typicalin the art.

In further embodiments, the IR-reflecting, preferably platelet-shaped,pigments of the invention are used preferably as a very opaque whitepigment in coating materials, preferably in colored coating materials,and very preferably in colored industrial coatings.

In the context of this use the core used is preferably a platelet-shapedaluminum pigment having an average size of 5 to 12 μm, the white pigmentused is preferably TiO₂, ZnS and/or ZnO with a preferred diameter of 250to 320 nm, and the matrix used is preferably SiO₂. In the context ofapplication in an industrial coating, a primary place is occupied notonly by the IR-reflecting properties of the pigment but also, moreparticularly, by the outstanding opacity of the platelet-shaped aluminumcore in the optical range.

Colored industrial coatings are often pigmented with large quantities ofexpensive colored pigments, but on account of the transparency of thosepigments possess inadequate opacity. The addition of white pigments suchas TiO₂ does improve the opacity, but inevitably leads to a lightershade. If the pigments of the invention are added to an industrialcoating, then, advantageously, the opacity can be distinctly improvedeven at very low levels of pigmentation, i.e., with small quantitiesadded, without having to accept any substantial lightening of thecoating. Hence the pigments of the invention, for applications in avarnish or industrial coating, are used at pigmentation levels of 0.1%to 4% by weight, preferably of 0.2% to 1.5% by weight, and morepreferably of 0.3% to 1.0% by weight, based in each case on the weightof the total formulation.

The examples which follow serve to elucidate the invention, withoutrestricting it in any way whatsoever.

EXAMPLES Inventive Example 1

100 g of aluminum powder (Reflexal 145, d₅₀=145 μm) are dispersed withstirring in 250 ml of isopropanol and the solvent is brought to boilingunder reflux. Then 17 g of tetraethoxysilane and, 2 minutes later 1.7 gof ethylenediamine in 50 g of fully demineralized water are added. In aseparate vessel, 200 g of TiO₂ pigment (Kronos 2310, average primaryparticle size: 300 nm; Kronos Titan, Friedrichstadt, Germany) aredispersed with stirring in 50 g of tetraethoxysilane. After a reactiontime of one hour, this dispersion is added continuously over the courseof an hour to the aluminum pigment suspension. Half an hour later, 1 gof ethylenediamine in solution in 20 g of isopropanol is added. One hourlater, 2.5 g of ethylenediamine in solution in 20 g of isopropanol areadded. The reaction mixture is left to boil for a further 4 hours andthen cooled to room temperature. The next day, the aluminum pigmentcoated with SiO₂ and the white pigment is filtered off on a Büchnerfunnel, washed repeatedly with isopropanol, and dried in an oven at 80°C. under reduced pressure.

Inventive Example 2

141 g of aluminum paste (Stapa Metalux 274, d₅₀=33 μm) are dispersedwith stirring in 250 ml of isopropanol and the solvent is brought toboiling under reflux. Then 17 g of tetraethoxysilane and, 2 minuteslater 1.7 g of ethylenediamine (EDA) in 50 g of fully demineralizedwater are added. In a separate vessel, 100 g of TiO₂ pigment (Kronos2310) are dispersed with stirring in 50 g of tetraethoxysilane. After areaction time of one hour, this dispersion is added continuously overthe course of an hour to the aluminum pigment suspension. Half an hourlater, 1 g of ethylenediamine in solution in 20 g of isopropanol isadded. One hour later, 2.5 g of ethylenediamine in solution in 20 g ofisopropanol are added. The reaction mixture is left to boil for afurther 4 hours and then cooled to room temperature. The next day, thealuminum pigment coated with SiO₂ and the white pigment is filtered offon a Büchner funnel, washed repeatedly with isopropanol, and dried in anoven at 80° C. under reduced pressure.

Comparative Example 3

Commercially available aluminum powder Reflexal 145, d₅₀=145 μm (ECKARTGmbH & Co. KG). Starting material for inventive example 1.

Comparative Example 4

Commercially available aluminum paste Stapa Metalux 274, d₅₀=33 μm(ECKART GmbH & Co. KG). Starting material for inventive example 2.

Comparative Example 5

Commercially available aluminum powder PCS 3500 (ECKART). This is anSiO₂-coated aluminum pigment with a very similar particle fraction toMEX 274 of comparative example 4.

Inventive Example 6

141 g of aluminum paste (Stapa Metalux 274, d₅₀=33 μm) are dispersedwith stirring in 250 ml of isopropanol and the solvent is brought toboiling under reflux. Then 17 g of tetraethoxysilane and, 2 minuteslater 1.7 g of ethylenediamine (EDA) in 50 g of fully demineralizedwater are added. In a separate vessel, 100 g of ZnS pigment (SachtolithL; Sachtleben; average particle size: 0.35 μm) are dispersed withstirring in 50 g of tetraethoxysilane. After a reaction time of onehour, this dispersion is added continuously over the course of an hourto the aluminum pigment suspension. Half an hour later, 1 g ofethylenediamine in solution in 20 g of isopropanol is added. One hourlater, 2.5 g of ethylenediamine in solution in 20 g of isopropanol areadded. The reaction mixture is left to boil for a further 4 hours andthen cooled to room temperature. The next day, the aluminum pigmentcoated with SiO₂ and the white pigment is filtered off on a Büchnerfunnel, washed repeatedly with isopropanol, and dried in an oven at 80°C. under reduced pressure.

Inventive Example 7

1 part of Standart® Reflexal 214 (ECKART GmbH & Co. KG) is incorporatedwith stirring into 4 parts of acetone, and then 1 part of a ground bulkpolymer based on methyl methacrylate (Degalan M 527; Degussa) and 1 partof Kronos 2310 are added and the mixture is stirred until the polymerhas dissolved completely.

The resulting suspension is sprayed via a spray-drying apparatus attemperatures above 60° C.

The resulting pigment is in the form of a white, nonlustrous powder.

Inventive Example 8

1 part of Standart® Reflexal 214 (ECKART GmbH & Co. KG) is incorporatedwith stirring into 4 parts of acetone, and then 1 part of a ground bulkpolymer based on methyl methacrylate and 1 part of ZnS (Sachtolith L;Sachtleben; average particle size: 0.35 μm) as white pigment are addedand the mixture is stirred until the polymer has dissolved completely.

The resulting suspension is sprayed via a spray-drying apparatus attemperatures above 60° C.

The resulting pigment is in the form of a white, nonlustrous powder.

Comparative Example 9

1 part of Standart® Reflexal 214 (ECKART GmbH & Co. KG) is mixed with 1part of Kronos 2310 white pigment by means of a centrifugal mixingassembly (DAC 400 FWC from Hausschild; Hamm) at 1000 rpm for 5 minutes.

The pigments of examples 1 to 9 were incorporated into an otherwiseunpigmented conventional varnish based on a polyester/CAB system(binders: 22% by weight CAB 381-2 and 9% by weight CAB 551-0.2, bothfrom Eastman, and 13% by weight Viacryl SC 303, fromSurfaceSpecialities). The level of pigmentation was in each case 10% byweight, based on the total varnish. Using a 50 μm coating knife,drawdowns were prepared and subjected to calorimetric measurement. Glossvalues were determined by means of the Tri-Gloss gloss meter fromByk-Gardner at 60° C., and the L,a,b values were determined at theobservation angles of 15°, 25°, 45°, and 110° C. (M 682, X-Rite). Thesevalues were used to calculate the flop index in accordance with formula(2) and, in conventional manner, the chroma at 250. The results areshown in Table 1:

TABLE 1 Colorimetric characteristics of the examples in knife drawdownsfrom a conventional unpigmented polyester/CAB system. Al DuPont fractionGloss flop in the Sample 60° L* 45° C* 25° index varnish Inventive 2.559.0 2.8 1.6 5.0 example 1 Inventive 10.9 65.9 1.8 0.9 5.0 example 2Comparative 41.6 37.1 5.1 16.5 10.0 example 3 Comparative 42.9 48.1 2.620.0 10.0 example 4 Comparative 40.1 46.9 2.5 20.3 10.0 example 5Inventive 10.8 63.7 0.5 1.6 5.0 example 6 Inventive 2.4 70.7 2.3 0.8 3.3example 7 Inventive 2.5 62.1 0.3 1.9 3.3 example 8 Comparative 13.9 69.71.6 3.7 5.0 example 9

All of the samples according to the invention have a similarly lowchroma C*25°, since the chroma of metal pigments is low per se onaccount of their achromaticity. In the lightness flop index and in thegloss, the inventive examples have much lower values in comparison tocomparative examples 3-5; the lightness flop, in particular, hasdisappeared almost completely. In contrast, in inventive examples, thelightness L*45° is much higher than in the case of comparative examples3-5. The reason for this is that a white pigment is generally associatedwith a relatively high lightness, and in the case of a metal pigment, atthis angle of observation, the lightness has already fallen distinctlyas a result of the flop.

Comparative example 9 has similar values for gloss, L*₄₅°, chroma, andlightness flop as the inventive examples. The degrees of gloss, however,are somewhat higher. The purely physical mixture of white pigments andaluminum pigments here has an apparently similar appearance to theinventive examples. Nevertheless, this mixture appears far less whiteand induces a greater “metallic” sensation in the observer.

In order to illustrate this, all of the pigments of examples 1 to 9 wereincorporated into the commercially available wall paint Krautol® atdifferent levels of pigmentation. An optically opaque knife drawdown wasproduced (100 μm knife depth) of each of these paints, and, as a blankspecimen, a knife drawdown without pigmentation with an IR-reflectingpigment. To determine the “whiteness” of the pigments, the lightness ofall of the drawdowns was measured in diffuse reflection geometry(Minolta CR 410, Minolta). For comparison, the uncoated aluminum pigmentand, respectively, an SiO₂-coated aluminum pigment of comparativeparticle size were incorporated into the wall paint (level ofpigmentation: 10% by weight). In the case of comparative example 4 itwas necessary to wash the pigment, which was in a white spirit paste,with acetone beforehand, since otherwise it was not possible toincorporate it into the aqueous wall paint. The results are shown inFIG. 1.

From FIG. 1 it is apparent that the lightness and therefore thewhiteness decrease as the concentration of the pigments goes up. Thelightnesses of the pure, uncoated pigments (comparative examples 3 and4) and of the pigments coated without white pigment (comparative example5) of the comparative examples are always smaller than those of theinventive examples. Of particular interest is the comparison of thelightnesses in light of the overall-identical metal content in the knifedrawdown. The arrows mark this comparison in each case. Here it isevident that, in the inventive examples, the pigments of the inventionconsistently possess a lightness higher by more than one unit. In visualterms, this is manifested in a significantly higher whiteness. Thisdifference, and also the visually perceived sparkle behavior, have beenset out briefly in Tab. 2. The visually assessed sparkle and whitesensations were evaluated in accordance with a five-point scale:

-   -   very strong    -   strong    -   moderate    -   weak    -   very weak

TABLE 2 Diffuse lightnesses, visual sparkle and white sensation inpigmented emulsion paint (whiteness) ΔL*_(diffuse) = L*_(diffuse,)L*_(diffuse) _(white pigment) − Visual Visual (10% metalL*_(diffuse, no) sparkle white Sample content) _(white pigment)sensation sensation Inventive 95.6 1.9 weak very example 1 strongComparative 93.7 0 very very example 3 strong strong Inventive 89.2 5.4very weak very example 2 strong Comparative 83.8 0.0 strong moderateexample 4 Comparative 85.0 1.2 strong moderate example 5 Inventive 89.15.3 weak strong example 6 Inventive 88.4 4.6 very weak strong example 7Inventive 88.1 4.3 weak strong example 8 Comparative 88.5 4.7 strongweak example 9

The visually assessed sparkle sensation is weakly or very weaklypronounced in the case of the pigments of the inventive examples. In theknife drawdowns, in contrast, the non-white-colored metal pigments ofthe comparative examples exhibit a pronounced sparkle behavior. This isbecause they are very large pigments, which can be perceivedindividually by the human eye within a paint. The same is true of thecomparative example 9, which in the calorimetric characterizations stillshowed very similar values to the inventive examples.

The calculated lightness differences are above a value of 1.5 in thecase of the pigments of the invention of the inventive examples. Thepigment of comparative example 5 (pigments with SiO₂ coating) likewiseshow a positive ΔL* value in comparison to the completely uncoated metalpigment, but the whiteness is not quite as high.

The ΔL* values measured appear low, but the human eye is very sensitivespecifically to the sensation of a white impression. In visual terms,therefore, it is possible to perceive very distinct differences betweenthe emulsion paint pigmented with the pigments of the invention and theemulsion paint pigmented with the uncoated pigments.

Hence the visually assessed whiteness of the inventive examples isconsistently strong to very strong. The comparative example 9, incontrast, shows only a moderate whiteness. The graying tendency of theuncoated aluminum pigments is manifested more strongly here. The sameapplies to comparative examples 4 and 5, while 2 evokes a very strongwhite sensation. In that case the very coarse aluminum pigment appearsto possess little graying tendency on account of its poor opacity.

Measurement of the Diffuse IR Reflection:

In the case of the pigments of the examples (except for comparativeexample 5), IR spectra were measured in diffuse reflection. For thispurpose, first of all, KBr powder was comminuted in a mortar. Thenpigment was added to the KBr in a concentration of 1.5% by weight andthe constituents were mixed homogeneously with one another. Atablet-shaped sample chamber (diameter: about 0.8 cm, depth: about 2.2mm) was filled with the KBr/pigment mixture, which was tamped down.Subsequently the diffuse reflection was measured in a wavelength rangefrom 2.5 to 25 μm. This was done using, as a measuring unit, theSelector (Specac). This instrument measures the diffuse IR reflection ina quarter-sphere geometry. The IR instrument used was an Avatac 360spectrometer from Thermo; the detector was a DTGS detector. In each casea pure KBr powder was measured as the background spectrum, and thespectrum of the KBr/pigment mixture was compared against it. The processis repeated three times and the average of the measurements is taken.

FIGS. 2 and 3 depict the spectra of a number of inventive andcomparative examples and plotted additionally (without scale) is thecalculated Planck radiation function at 300 K. In FIG. 2 the spectra ofthe inventive and comparative examples are shown with coarse pigments(D₅₀=about 145 μm), and in FIG. 3 with the finer pigments (D₅₀=about 35μm).

A comparison of the spectra of FIG. 2 shows that the pigments ofinventive example 1 possess a lower reflection than the uncoated metalpigment of comparative example 3. This can be attributed to a certaindegree of IR absorption by the TiO₂ pigments and the SiO₂ matrix of thecoating. Overall, nevertheless, the reflection is high enough to producean IR-reflecting pigment. Similar circumstances can be observed in FIG.3 for the finer pigments. In this case the reflection and thereflectance, more particularly for inventive example 6, aresignificantly higher than for examples 2 or 4. This can be attributed tothe low IR absorption of ZnS pigments in comparison to TiO₂ pigments.

This can be understood by reference to Tab. 3. Here, from the spectralIR reflection spectra obtained, the reflectance the IR reflectivity wascalculated by formula (1), using a temperature of 300 K for the Planckfunction.

TABLE 3 Evaluation of diffuse IR spectra IR reflectance (2.5 to 25 μm)IR reflectance, Sample [%] coating Inventive 63.4 71.5% example 1Inventive 62.3 76.6% example 2 Comparative 88.7  100% example 3Comparative 81.3  100% example 4 Comparative / / example 5 Inventive79.8 98.2% example 6 Inventive / / example 7 Inventive / / example 8Inventive 59.7 73.4% example 9

The pigments of the inventive examples possess an IR reflectance ofsignificantly more than 50%. The influence of the coatings on the IRreflectivity is relatively low, as may be inferred from the high valuesof more than 71% for the IR reflectance, coating. Particularly in thecase of inventive example 6 the influence of the coating is only verysmall.

The advantages of the pigment of the invention are therefore to be seenin a joint viewing of high IR reflectivity on the one hand and whiteimpression on the other hand.

The pigments of comparative examples 3 and 4 possess even higherreflectances, since there are no coatings present. On account of theirgraying effects and their distinct sparkle effect, however, thesepigments cannot be used in a wall paint. In the case of comparativeexample 9, in contrast, the reflectance is relatively small, since herethe TiO₂ added to the mixture effects absorption.

Consequently, in a joint consideration of high IR reflectivity, asubstantially white appearance, and the absence of characteristicsspecific to effect pigment, such as gloss and lightness flop, theIR-reflecting pigments of the invention display advantages in relationto mixtures of uncoated effect pigments and white emulsion paints.

1. A pigment which reflects IR radiation, comprising an IR-reflectingcore, wherein the IR-reflecting core is provided with a substantiallyenveloping coating which is substantially transparent to IR radiationand in that the IR-reflecting pigment is substantially white.
 2. TheIR-reflecting pigment of claim 1, wherein the IR-reflecting core isplatelet-shaped or spherical.
 3. The IR-reflecting pigment of claim 1,wherein in, on and/or under the coating that is substantiallytransparent to IR radiation there are white pigments which preferablyhave an average primary particle size of 180 to 400 nm.
 4. TheIR-reflecting pigment of claim 3, wherein the white pigments have anaverage primary particle size of 250 to 370 nm.
 5. The IR-reflectingpigment of claim 3, wherein the white pigments are selected from thegroup consisting of titanium dioxide, zinc oxide, magnesium oxide, zincsulfide, calcium fluoride, lithium fluoride, sodium fluoride, potassiumfluoride, calcium carbonate, lithopones, magnesium carbonate, bariumsulfate, barium titanate, barium ferrite, and mixtures thereof.
 6. TheIR-reflecting pigment of claim 3, wherein the white pigments are TiO₂,ZnS and/or ZnO.
 7. The IR-reflecting pigment of claim 1, wherein thewhite pigments are present in an amount of 20% to 80% by weight,preferably 35% to 70% by weight, based on the weight of the totalIR-reflecting pigment.
 8. The IR-reflecting pigment of claim 1, whereinthe white pigments are arranged substantially uniformly around theIR-reflecting core and are present preferably in an amount of 0.3 to 10g, preferably of 0.5 to 7 g, per 1 m² of surface area of theIR-reflecting core in the IR-reflecting pigment.
 9. The IR-reflectingpigment of claim 1, wherein the substantially IR-transparent coatingcomprises or consists of metal oxide.
 10. The IR-reflecting pigment ofclaim 9, wherein the metal oxide is selected from the group consistingof silicon dioxide, aluminum oxide, aluminum hydroxide, boron oxide,boron hydroxide, and mixtures thereof.
 11. The IR-reflecting pigment ofclaim 1, wherein the substantially IR-transparent coating is present ina fraction of 4% to 30% by weight, based on the weight of the totalIR-reflecting pigment.
 12. The IR-reflecting pigment of claim 1, whereinthe IR-reflecting core is a platelet-shaped metal pigment or a sphericalmetal particle, the metal being selected preferably from the groupconsisting of aluminum, copper, zinc, iron, silver, and alloys thereof.13. The IR-reflecting pigment of claim 12, wherein the IR-reflectingcore is a platelet-shaped metal pigment having a size of 5 to 150 μm.14. The IR-reflecting pigment of claim 12, wherein the IR-reflectingcore is a platelet-shaped metal pigment having a size of 5 to 12 μm. 15.The IR-reflecting pigment of claim 12, wherein the IR-reflecting core isa platelet-shaped aluminum pigment.
 16. The IR-reflecting pigment ofclaim 12, wherein the IR-reflecting core is platelet-shaped aluminumpigment, the IR-transparent coating is SiO₂, and the white pigment ispreferably TiO₂, ZnS and/or ZnO having an average primary particle sizeof preferably 250 to 320 nm.
 17. The IR-reflecting pigment of claim 1,wherein the IR-transparent coating is or comprises organic polymer. 18.The IR-reflecting pigment of claim 17, wherein the IR-transparentcoating is present in a fraction of 4% to 30% by weight, based on theweight of the total IR-reflecting pigment.
 19. The IR-reflecting pigmentof claim 17, wherein the IR-reflecting core is a platelet-shaped metalpigment or a spherical metal particle, the metal being selectedpreferably from the group consisting of aluminum, copper, zinc, iron,silver, and alloys thereof.
 20. The IR-reflecting pigment of claim 19,wherein the IR-reflecting core is a platelet-shaped metal pigment havinga size of 5 to 150 μm.
 21. The IR-reflecting pigment of claim 19,wherein the IR-reflecting core is a platelet-shaped metal pigment havinga size of 5 to 12 μm.
 22. The IR-reflecting pigment of claim 19, whereinthe IR-reflecting core is a platelet-shaped aluminum pigment.
 23. TheIR-reflecting pigment of claim 1, wherein the IR-reflecting core is apearlescent pigment which, preferably largely, reflects IR radiationfrom 6 to 14 μm.
 24. A process for producing an IR-reflecting pigment ofclaim 1, wherein a coating which is substantially transparent to IRradiation is applied, together with white pigments and/or withparticlelike coating outgrowths that scatter visible light, to anIR-reflecting core.
 25. The process for producing an IR-reflectingpigment of claim 24, wherein white pigments are applied, together withmetal oxide, to the IR-reflecting core by a wet-chemical sol-gelprocess.
 26. The process for producing an IR-reflecting pigment of claim25, wherein SiO₂ as metal oxide is applied to the IR-reflecting core bya wet-chemical sol-gel process.
 27. The process for producing anIR-reflecting pigment of claim 24, wherein a dispersion comprisinghighly volatile organic solvent, IR-reflecting cores, white pigments,preferably with an average size of 180 to 400 nm, and organicfilm-forming agent is spray-dried.
 28. The use of an IR-reflectingpigment of claim 1 in inks, paints, varnishes, printing inks,security-printing inks, and cosmetics.
 29. The use of an IR-reflectingpigment of claim 1 in emulsion paints for the interior or exteriorsector.
 30. A coating composition wherein the coating compositioncomprises an IR-reflecting pigment of claim
 1. 31. The coatingcomposition of claim 30, wherein the coating composition is a cosmetic,varnish, paint or ink, more particularly printing ink, security-printingink or emulsion paint.
 32. An article wherein the article is coated withan IR-reflecting pigment of or with a coating composition of claim 30.